The invention relates, in general, to non-volatile memory devices and, more particularly, to a non-volatile memory device having a Polysilicon Oxide Nitride Oxide Semiconductor (SONOS) structure in which a charge trap layer is separated physically horizontally and a method of manufacturing the same.
In recent years, Non-volatile Semiconductor Memories (NVSM) are largely classified into floating gate series and Metal Insulator Semiconductor (MIS) series in which two or more kinds of dielectric layers are laminated doubly or triply in terms of the process technology.
The floating gate series implement a memory characteristic by employing the potential well. A representative example of the floating gate series is an EPROM Tunnel Oxide (ETO) structure that has been widely used as flash Electrically Erasable Programmable Read Only Memory (EEPROM). The MIS series performs the memory function employing traps existing at the dielectric layer bulk, the dielectric layer dielectric layer interface, and the dielectric layer-semiconductor interface. A representative example of the MIS series is a Metal/Polysilicon Oxide Nitride Oxide Semiconductor (MONOS/SONOS) that has been widely used as flash EEPROM.
A difference between the SONOS and general flash memory is that in the general flash memory, charges are stored in the floating gate, whereas in the SONOS, charges are stored in the nitride layer in terms of the structure.
Furthermore, in the general flash memory, the floating gate is formed using polysilicon. Thus, if any one defect exists in polysilicon, the retention time of charge is significantly lowered. In contrast, in the SONOS, the nitride layer is used instead of polysilicon as described above. Accordingly, the sensitivity to defect in process is relatively small.
In addition, in the general flash memory, tunnel oxide having a thickness of about 70 Å is formed under the floating gate. There is a limit to the implementation of a low-voltage and high-speed operation. However, in SONOS, direct tunneling oxide is formed under the nitride layer. It is therefore possible to implement a memory device having a lower voltage, lower power and high-speed operation.
A conventional flash memory device having the SONOS structure will be described below with reference to FIG. 1.
Referring to FIG. 1, a tunnel oxide layer 11, a charge trap layer 12, a blocking gate (oxide layer) 13, and an electrode 14 for a gate are sequentially formed on a semiconductor substrate 10. Word line patterns are then formed by an etch process. In the flash memory cell of the SONOS structure, since the same E-field is applied to the whole complex layer of the blocking oxide layer 13 (i.e., an insulating layer), the charge trap layer 12 for storing charges and the tunnel oxide layer 11, different electric field (E-field) cannot be applied to the insulating layers, respectively.
In this case, if a voltage is applied to the electrode 14 for gate in order to erase the charges stored in the charge trap layer 12, the charges stored in the nitride layer 12 are moved toward the semiconductor substrate 10 by means of a Fowler-Nordheim (F—N) tunneling current through the tunnel oxide layer 11 and are then erased.
As the cell size of a memory cell is reduced, it becomes difficult to separate charges trapped at the source region 15 and the drain region 16 in the cell of the SONOS structure. If the density of charges is increased in order to implement a multi-level, the interference phenomenon becomes more profound, which leads to limit the level of integration.